Device and method for siezing,sizing, sifting, filtering or sorting substances

ABSTRACT

Particles (P) contained in a liquid are sieved by placing them onto a sieving surface ( 3 ) located inside a sieving frame ( 2 ). The sieving process ensues by subjecting the liquid (F) to ultrasonic oscillations (U). The ultrasonic oscillations (U) are generated using an ultrasonic device ( 4 ) having an ultrasonic irradiation surface ( 5 ) that can be brought into contact with the liquid (F). At least one ultrasonic transducer ( 6 ) is arranged on the side ( 7 ) of said ultrasonic irradiation surface that faces away from the liquid. The ultrasonic device ( 4 ) comprises an approximately conical ultrasonic irradiation surface ( 5 ).

The invention relates to a device and a process for screening,classifying, sifting, filtering or sorting of substances with thefeatures of the preamble of the independent claims.

It is known that ultrasonic vibrations benefit screen feed in ascreening machine. The vibrating motions can reduce agglomeration forcesand surface tensions and thus reduce or prevent the danger of cloggingof the screening meshes.

WO 94/27748 discloses for example providing a screening cloth with aresonator with finger-like resonator rods so that ultrasonic vibrationsare distributed as much as possible over the entire screening cloth.While this arrangement works satisfactorily in conjunction with dryscreen feed, problems arise to some extent in screening of particlesheld in liquids. It is especially due to the fact that the liquidcomparatively strongly attenuates the ultrasonic vibrations on the clothso that in spite of the resonator rods parts of the screening cloth arenot exposed to vibrations. The designations screening/screen are used ina standard manner here and hereinafter, and any treatment of thematerial, especially also classification, sifting, filtering or sortingare to be encompassed by this concept.

Various devices are known for screening of particles held in liquidswith ultrasound.

U.S. Pat. No. 4,693,879 teaches mounting an ultrasonic sonotrodeadjacent to the surface of a revolving screen. One disadvantage in thisarrangement is that the entire screen surface cannot be exposed toultrasound. For this reason only one part of the screen surface at atime can be actively used.

U.S. Pat. No. 4,282,100 likewise proposes use of a revolving screen. Thematerial to be screened is routed through a gap between the sonotrodeand the screen surface. Here too only a limited part of the screensurface can be actively used.

U.S. Pat. No. 3,490,584 shows a conventional screening machine in whichthe liquid held on the screening cloth is exposed to ultrasonicvibrations by means of an ultrasonic sonotrode. As a result of thelimited dimensions of the sonotrode it is however necessary to rotatethe screening machine in order to expose the entire screen surface toultrasonic vibrations. The screening machine with a rotary drive istherefore complex in its construction.

U.S. Pat. No. 3,756,400 discloses inserting an ultrasonic sonotrode intoa tubular container in which the liquid to be screened and the screeningcloth are contained. One disadvantage in this arrangement is that as aresult of the limited cross section of the sonotrode only relativelysmall screen surfaces can be used.

The object of this invention is to avoid the disadvantages of what isknown, especially therefore to devise a device and a process forscreening of particles which are contained in the liquid, with which ina structurally simple manner essentially the entire screen surface canbe exposed to ultrasound. In doing so if possible rotating screenarrangements will be avoided. The device as claimed in the inventionshould moreover be easily applicable to existing screening machines.Another object of the invention is to increase the efficiency ofexposure to ultrasound. Still another object is to increase theultrasonic energy which can be delivered into the liquid. The devicewill moreover allow reliable operation; especially the destruction ofthe ultrasonic arrangement by overload, for example with overly smallamounts of the screen feed, will be avoided.

These objects are achieved as claimed in the invention with a device anda process with the features of the characterizing part of theindependent claims.

The device for screening of particles contained in a liquid has at leastone screen frame with a screen surface. The liquid with the particles tobe screened can be applied to the screen surface. The device moreoverhas at least one ultrasonic arrangement for delivering ultrasonicvibrations into the liquid. The ultrasonic arrangement has an acousticirradiation surface which can be brought into contact with the liquid onthe screen surface. The acoustic irradiation surface can be caused tovibrate with at least one ultrasonic transducer. The ultrasonictransducer is preferably located on the side of the acoustic irradiationsurface facing away from the liquid.

As claimed in the invention the ultrasonic arrangement is therefore madeas a so-called immersible transducer. One or more ultrasonic transducersproduce ultrasonic vibrations on an acoustic irradiation surface whichis typically made as a sheet. The acoustic irradiation surface whichvibrates overall can be brought into contact with the liquid whichcontains particles. Thus essentially the entire liquid contained on thescreen surface can be exposed to sonic waves. In doing so the liquid isexposed to sonic waves directly from overhead, i.e. not via the screensurface. With these immersible transducers it is moreover possible todeliver much higher ultrasonic energies into the liquid than withultrasonic sonotrodes, typically up to 10 kW.

Because the liquid with the particles which are to be screened is to acertain extent squeezed between the screen surface and the acousticirradiation surface, in addition a static pressure forms which can havea positive effect on the screening properties.

According to one preferred embodiment therefore the acoustic irradiationsurface covers essentially the entire screen surface.

The screen surface is typically made roughly round (as in conventionalscreening machines). The ultrasonic arrangement is made roughlyrotationally symmetrical with respect to the axis perpendicularly to thescreen surface. Typically the ultrasonic arrangement can be madehexagonal.

Advantageously the ultrasonic arrangement has a feed opening. The feedopening is preferably arranged centrally. This makes it possible to feedthe liquid roughly into the center of the screen surface through theultrasonic arrangement located above the screen surface.

According to one especially preferred embodiment a gap is formed betweenthe screen surface and the acoustic irradiation surface; the distance ofthe gap from the feed opening for the liquid with the particlesdiminishes toward a discharge opening for material which cannot bescreened. In the operation of the device, as a result of the newlysupplied liquid, the liquid to be screened is moved from the feedopening towards the discharge opening. The angle between the acousticirradiation surface and the screen surface and/or the width of the gapcan be made adjustable.

Due to the vibrating motion of the screening machine the screen feed isconveyed to the outside by centrifugal forces. The static pressure ofthe screen feed slightly compresses it as the gap width is reduced. Thisensures that the acoustic irradiation surface always remains in contactwith the screen feed.

A radially symmetrical ultrasonic arrangement is especially preferred inwhich the gap between the screen surface and the acoustic irradiationsurface decreases continuously from the center of the ultrasonicarrangement towards the outer edge. The acoustic irradiation surface istherefore made roughly conical, and for structural reasons a structureof flat component pieces is conceivable.

Advantageously the screen frame is provided with at least one lateraldischarge opening for material which cannot be screened. Material whichhas not yet been screened when the edge of the screen is reached isdischarged through the discharge opening, to a certain extent scoured bythe liquid. Thus continuous operation of the device as claimed in theinvention is possible.

So that the ultrasonic arrangement can be used on conventional screeningmachines, it is advantageously made for detachable connection to theoutside wall (top cover) of a conventional screening machine.

In order to prevent overloading of the ultrasonic arrangement, thedevice can be provided preferably with means for measuringand/controlling the level of the liquid on the screen surface. Thus itis possible to prevent the acoustic irradiation surface from movingcompletely or partially out of contact with the liquid and thus undampedoperation from occurring. To measure the level a measurement sensor canbe used. The level however can be determined indirectly especiallyadvantageously via the load of the generator for the ultrasonicarrangement.

According to one preferred embodiment a so-called static screeningmachine can be used. The screening cloth is located at an angle to thehorizontal (in the operating position of the screen feed) so that thescreen feed moves down over the screen surface as a result ofgravitation.

The acoustic irradiation surface is located at an angle to the screensurface. The distance between the acoustic irradiation surface andscreen surface decreases downward.

In this embodiment there can be one or more ultrasonic arrangements.

The acoustic irradiation surfaces are each made flat, and the angleand/or the distance between the acoustic irradiation surface and thescreen surface can also be adjustable.

According to another alternative embodiment the screen surface of thedevice can be advantageously located to be movable relative to at leastone ultrasonic arrangement. The acoustic irradiation surface canpreferably be located at an angle to the screen surface. In this way thewidth of the gap between the screen surface and the acoustic irradiationsurface decreases from material feed to material discharge. The screensurface can advantageously be moved in the direction between materialfeed and material discharge. This yields various advantages. By movingthe screen surface and thus also the screen feed in the direction of thenarrowing gap, pressure is additionally produced on the screen feed. Inthis way the ultrasonic action is increased. At the same time largeparticles which cannot be screened are discharged by the motion of thescreen. Accumulation of particles which cannot be screened is thisprevented.

It is advantageous to make the screen surface as a closed, flexiblescreen which is clamped on two rotationally supported rollers.

Therefore the invention consists among others in using an immersibletransducer (known for example from ultrasonic cleaning) for exposing thescreen feed in liquids to sonic waves. In this connection liquid withparticles is defined as all flowable media which contain particles to bescreened, therefore typically dispersions, and the actual liquid portioncan be so small that the liquid to be treated has a sludge-likeconsistency.

The invention is detailed below in embodiments and using drawings.

FIG. 1 shows a cross section through a device as claimed in theinvention,

FIG. 2 shows a perspective of an ultrasonic arrangement as claimed inthe invention,

FIG. 3 shows an overhead view of the ultrasonic arrangement as shown inFIG. 2,

FIG. 4 shows an enlargement of one extract from FIG. 1, and

FIGS. 5 and 6 show schematics of two alternative embodiments.

FIG. 1 shows a device 1 which is used for screening of particles P whichare contained in a liquid F. The device 1 has a screen frame 2 whichbears a screen surface 3. The screen surface 3 is typically a knownscreening cloth. The screen frame 2 and the screen surface 3 form partof a conventional screening machine 12. The screening machine 12 is ofconventional design and is made as a disk, vibration or shakingscreening machine.

Above the surface 3 as claimed in the invention there is a ultrasonicarrangement 4. The ultrasonic arrangement 4 is made like a conventionalimmersible transducer and has an acoustic irradiation surface 5. On theside 7 facing away from the acoustic irradiation surface there aretwelve ultrasonic transducers 6. The ultrasonic arrangement 4 is made asa closed box, with ultrasonic transducers 6 located on its inside 7 (seealso FIG. 4). The acoustic irradiation surface 5 of the ultrasonicarrangement 4 can be brought into contact with the liquid F whichcontains the particles P. In this way ultrasonic vibrations U can bedelivered into the liquid F via the acoustic irradiation surface 5.

The ultrasonic arrangement 4 is made rotationally-symmetrical withrespect to the axis A perpendicularly to the screen surface 3. Theconstruction is typically hexagonal (see FIG. 2 or 3). One suchhexagonal construction is recommended for structural reasons. Of coursealso round arrangements or polygonal arrangements with more or less thansix corners would be conceivable. In the middle the ultrasonicarrangement 4 is provided with a feed opening 8 by which material to bescreened can be applied to the screen surface 3.

The ultrasonic arrangement 4 is held in a round outside wall 18. Theoutside wall 18 (top cover) is part of the screening machine 12. In thescreen frame 2 or in the outside wall 18 there is an opening 9. MaterialM which has not been screened or which cannot be screened can bedischarged through the opening 9.

The acoustic irradiation surface 5 of the ultrasonic arrangement 4 istilted with respect to the screen surface 3 at an angle α of roughly 10°(see FIG. 4) so that between the screen surface 3 and the acousticirradiation surface 5 a narrowing gap 11 is formed. The distance abetween the screen surface 3 and the acoustic irradiation surface 5decreases continuously from the middle of the ultrasonic arrangement 4towards the outer edge 10. The distance a can also be adjustable, forexample by adjustable mounting of the ultrasonic arrangement 4 on thescreening machine 12.

During operation the material to be screened (liquid F with particles P)is delivered onto the screen surface 3 in the middle of the ultrasonicarrangement 4. Screenable material passes through the screen surface 3and is removed from the screening machine 12 by a drain 19. Materialwhich has not been screened or which cannot be screened moves radiallyto the outside as a result of the newly supplied screen feed andscreening machine vibrations. Material which has not yet been screenedwhen the outer edge 10 is reached is discharged through the dischargeopening 9.

The liquid F contained on the screen surface 3 has a certain level N. Toprevent the ultrasonic arrangement 4 from operating without a load andthus being destroyed, it must be ensured that the level N of the liquidF does not drop below a predetermined value. For this reason there is ameasurement sensor 12 which is shown schematically and which measuresthe level N. As soon as the level N drops below a setpoint, theultrasonic generator is stopped and/or addition screen feed is added.But it is also conceivable to determine the level N via the load of thegenerator for the ultrasonic transducer 6.

FIG. 2 shows the construction of an ultrasonic arrangement 4. Theultrasonic arrangement 4 is made as a box.

The rear wall 14 of the box-shaped ultrasonic arrangement 4 is providedmoreover with reinforcing or holding sheets 16 which are arranged in astar-shape around the supply pipe 15 for the screen feed (FIGS. 2 and3). The ultrasonic arrangement is preferably detachably connected to theoutside wall 18 via the holding sheets 16. The holding sheets 16 can beadjustably connected to the outside wall 18 so that the distance a canbe set. By choosing the size of the holding sheets 16, the ultrasonicarrangement can be matched to the dimension of the screen frame 2. Thebox-shaped ultrasonic arrangement 4 is welded tight against the outside.On the side 7 of the acoustic irradiation surface 5 facing away from theliquid F there are ultrasonic transducers 6 (see FIG. 1). The powersupply (not shown) for the ultrasonic transducer is routed out throughthe surface of the box (for example through the back).

The box-like ultrasonic arrangement 4 is bordered on the outside by sixside walls 13 and on the inside by six inside walls 17 (see FIGS. 3 and4). As a continuation of the round feed opening 8 on the back wall, thefeed pipe 15 is on the back wall 14, by which pipe the material to bescreened can be introduced.

The box contains six component surfaces 20 which together form theacoustic irradiation surface 5 (see FIGS. 3 and 4). The componentsurfaces 20 are each made trapezoidal. Two component surfaces 20 next toone another at a time are welded to one another along their sides, thecomponent surfaces 20 not lying in the same plane so that the acousticirradiation surface 5 has an overall somewhat conical shape (see FIG.4).

FIG. 3 shows the hexagonal execution of the ultrasonic arrangement 4.The holding sheets 16 join the arrangement 4 to the outside wall 18. Theoutside wall is made round and forms part of the screening machine. Theultrasonic arrangement 4 has a somewhat smaller dimension than thescreen frame 2 and is bordered on the outside by the side walls 13 andis made hexagonal. The round feed opening 8 in the back wall 14 of theultrasonic device 4 is made smaller relative to the inside walls 17. Theinside walls 17 together likewise form a hexagon.

FIG. 4 shows an enlarged extract from FIG. 1, for reasons of clarity theliquid F with the particles P which are to be screened not being shown.

FIG. 4 shows the arrangement of the individual component surfaces 20 invarious planes which leads to a somewhat conical shape of the acousticirradiation surface 5.

The ultrasonic transducers 6 is typically twelve piezoelectrictransducers. The piezoelectric transducers 6 are cemented to the side 7of the ultrasonic arrangement 4 facing away from the liquid. Theultrasonic transducers 6 are operated at a frequency of roughly 30 kHzparallel to one another with a conventional ultrasonic generator. Apower of up to 0.6 kW can be delivered into the liquid F with the twelvetransducers 6 shown here.

The box-shaped ultrasonic arrangement 4 consists of parts of stainlesschromium steel (1.4301) welded to one another, with a thickness oftypically 2 mm.

FIG. 5 schematically shows one alternative embodiment. The device ismade as a static screening machine 31. A screen surface 31 is clamped onthe screen frame 32 at an angle γ to the horizontal H. Above the screensurface 33 there are two ultrasonic arrangements 34. The liquid F to bescreened, with particles P, is routed between the acoustic irradiationsurface 35 of the ultrasonic arrangement 34 and the screen surface 33.The ultrasonic arrangements 34 are arranged to be adjustable so that theangles β, β′ between the acoustic irradiation surface 35 and the screensurface 33 can be adjusted, for example depending on the screen feed.FIG. 5 shows two ultrasonic arrangements 34. The material to be screenedmoves by gravity over the screen surface 33 from the feed opening 38down to a discharge opening 39 and is exposed to ultrasonic vibrations Uin doing so. The width b of the gap 41 between the screen surface 33 andthe acoustic irradiation surface 35 decreases downward. The ultrasonicarrangement 34 which is the upper in FIG. 5 is located at a distance tothe screen surface 33 so that unscreened material can move farther alongthe screen surface 3 where it is exposed to vibrations by the ultrasonicarrangement 34 which is the bottom one in FIG. 5.

The angles β, β′ can be set individually and can typically be 5-15°.

FIG. 6 shows another alternative embodiment of the device as claimed inthe invention. The device 51 has two rotationally mounted rollers 52. Aclosed flexible screen 53 is clamped on the rollers 52. By turning therollers the screen 53 can be moved in the direction R with a speed ofroughly 0.5-3 m/min. The particles P to be screened in the liquid F aredelivered to the screen 53 at the material feed 58. As a result of themotion of the screen 53 the liquid F is conveyed with particles P in thedirection of the ultrasonic arrangements 54 which are provided withacoustic irradiation surfaces 55. Between the acoustic irradiationsurfaces 55 and the screen surface 53 a gap is formed with a width whichdecreases in the direction R from the material feed 58 to the materialdischarge 59. Material M which cannot be screened is removed as a resultof the motion of the screen surface 53 at the material discharge 59 fromthe screen surface 53. In addition, there can be stripping mechanismssuch as doctor blades which remove the particles M which cannot bescreened from the screen surface 53.

The screened material is captured underneath the screen in a trough. Thetrough can be tilted for example so that the screened material flowsaway.

1-22. (canceled)
 23. A device for screening, classifying, sifting,filtering or sorting particles contained in a liquid, said devicecomprising at least one screen frame having a screen surface to whichthe liquid containing the particles can be applied, at least oneultrasonic arrangement for delivering ultrasonic vibrations into theliquid, wherein the ultrasonic arrangement has an acoustic irradiationsurface which can be brought into contact with the liquid on the screensurface and at least one ultrasonic transducer for vibrating saidsurface, at least the ultrasonic transducer being located on a sidefacing away from the liquid.
 24. A device as claimed in claim 23,wherein substantially all of the screen surface is covered by theacoustic irradiation surface.
 25. A device as claimed in claim 23,wherein the screen surface is substantially round and the ultrasonicarrangement is made substantially rotationally symmetrical with respectto an axis perpendicular to the screen surface.
 26. A device as claimedin claim 23, wherein the ultrasonic arrangement has a feed opening forthe liquid.
 27. A device as claimed in claim 26, wherein a gap is formedbetween the screen surface and the acoustic irradiation surface, the gaphaving a width decreasing away from the feed opening for the liquid,toward at least one discharge opening for material which cannot bescreened.
 28. A device as claimed in claim 27, wherein the gap betweenthe screen surface and the acoustic irradiation surface has a widthwhich decreases from the middle of the ultrasonic arrangement radiallytowards the outer edge.
 29. A device as claimed in claim 27, wherein thedischarge opening is made as an opening in the peripheral outside wallof the ultrasonic arrangement.
 30. A device as claimed in claim 23,wherein the ultrasonic arrangement is or can be detachably connected toa screening machine.
 31. A device as claimed in claim 23, furthercomprising means for measuring or controlling liquid level on the screensurface.
 32. A device as claimed in claim 23, wherein the screen surfaceis tilted relative to the horizontal and wherein the acousticirradiation surface is flat and runs at an angle to the screen surface.33. A device as claimed in claim 23, wherein the distance and/or theangle between the acoustic irradiation surface and the screen surface isadjustable.
 34. A device as claimed in claim 23, wherein the screensurface is movable relative to said at least one ultrasonic arrangement.35. A device as claimed in claim 34, wherein the acoustic irradiationsurface is located at an angle to the screen surface so that the widthof the gap between the screen surface and the acoustic irradiationsurface decreases from material feed to material discharge, the screensurface being movable in a direction between material feed and materialdischarge.
 36. A device as claimed in claim 34, wherein the screensurface is a closed, flexible screen which is clamped on tworotationally supported rollers.
 37. An ultrasonic arrangement for adevice as claimed in claim 23, wherein the ultrasonic arrangement has anacoustic irradiation surface which can be brought into contact with theliquid,
 38. An ultrasonic arrangement as claimed in claim 37, whereinthe ultrasonic arrangement is seated on the screen frame of aconventional screening machine.
 39. An ultrasonic arrangement as claimedin claim 37, wherein the acoustic irradiation surface runs roughlyconically, and is made of component surfaces which are at an angle toone another.
 40. A process for screening, classifying, sifting,filtering or sorting of particles contained in a liquid on a screensurface, said process comprising steps of delivering ultrasonicvibrations into the liquid by means of an ultrasonic arrangement havingan acoustic irradiation surface which can be brought into contact withthe liquid, and vibrating the acoustic irradiation surface with at leastone ultrasonic transducer located on a side of the acoustic irradiationsurface facing away from the liquid.
 41. A process as claimed in claim40, wherein the liquid with the particles is routed through a gapbetween the acoustic irradiation surface and the screen surface.
 42. Aprocess as claimed in claims 40, wherein the level of the liquid on thescreen surface is measured and/or controlled.
 43. A process as claimedin claims 40, wherein the screen surface is moved during the screeningprocess.
 44. A process as claimed in claim 43, wherein the screensurface is moved from the material feed in the direction of the materialdischarge.